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Milky Way star set to go supernova

One of the most powerful explosions in the cosmos is set to take place in our own neighbourhood. Lauren Fuge reports.

An animation showing the tumultuous interaction of gas and dust around the star astronomers think may soon go nova.

University of Sydney

Brace yourself, because a star in our galaxy is set to explode in one of the most energetic events in the universe.

An international team of astronomers, led by Joe Callingham from the Netherlands Institute for Radio Astronomy (ASTRON), recently discovered a pair of hot, luminous stars about 8000 light-years from Earth – and one is teetering on the edge of supernova.

The team predicts this will produce a gamma-ray burst, making the star the first known candidate for such an event found in the Milky Way.

Gamma-ray bursts are intense flashes of high-energy radiation, lasting anywhere from a fraction of a second up to a few minutes. They are the most powerful explosions in the cosmos and the most luminous light sources other than the Big Bang. At their peak, they can briefly outshine the entire universe.

Crucially, each burst can be observed only once – and whatever produces it is destroyed or altered beyond recognition. Decades of observations and theoretical models have led astronomers to believe that gamma-ray bursts are produced by either the collision of two compact objects (such as two neutron stars in a binary system) or the collapse of a single massive star.

Both mechanisms trigger the formation of a black hole surrounded by a rapidly spinning disk of material. As the black hole guzzles down matter, two energetic jets of high energy gamma rays are thought to be created.

This newly-discovered star – nicknamed Apep after the serpentine Egyptian god of chaos – falls into the latter category.

Both stars in the system are classified as Wolf-Rayet stars, which are massive, hot stars that are rapidly approaching the end of their life. As they orbit each other every hundred years or so, powerful stellar winds stream off them both, forming elegant coils of dust.

“When we saw the spiral dust tail we immediately knew we were dealing with a rare and special kind of nebula called a pinwheel,” says co-author Peter Tuthill, an astronomer at the University of Sydney in Australia.

“The curved tail is formed by the orbiting binary stars at the centre, which inject dust into the expanding wind, creating a pattern like a rotating lawn sprinkler. Because the wind expands so much, it inflates the tiny coils of dust revealing the physics of the stars at the heart of the system.”

But the team soon faced a puzzle. Using the European Southern Observatory’s Very Large Telescope in Chile and the Anglo-Australian Telescope in Australia, they measured the speed of the stellar winds to be 12 million kilometres per hour – mind-blowingly fast.

But the dust seeded from this gas was moving much slower, at only two million kilometres an hour.

“It was like finding a feather caught in a hurricane just drifting along at walking pace,” Tuthill says.

A composite image showing the two stars in the target system orbiting each other.

University of Sydney/European Southern Observatory

Benjamin Pope, co-author from New York University, explains: “The only way we get such a system to work is if the Wolf-Rayet star is spewing out gas at several speeds.”

At its poles Apep is emitting fast, hot gas, but from its equator the wind is slower and denser. Such winds are known to the product of fast-rotating stars – Apep is spinning so fast it’s about to rip itself apart.

“The rapid rotation puts Apep into a whole new class,” explains Pope. “Normal supernovae are already extreme events but adding rotation to the mix can really throw gasoline on the fire.”

This could create the perfect stellar storm – when Apep finally collapses, it may trigger a gamma-ray burst.

Previously, astronomers have only witnessed long-duration gamma-ray bursts in distant galaxies. They didn’t expect to see one in the Milky Way because all massive stars here are metal-rich, and it’s thought that to form a gamma-ray burst, Wolf-Rayet stars must be fast-spinning and low in metal content.

“This is because the metals carry away a lot of angular momentum in the star’s wind as it loses mass, essentially acting as a really efficient brake for the rotation of the star,” lead author Callingham says.

So by the time a metal-rich Wolf-Rayet star dies, it is rotating much slower and can’t trigger a gamma-ray burst.

But Apep might be the exception – indeed, it might be a prototype for a new class of such stars.

“Even though Apep likely has high-metallicity, the interaction with the binary companion could be the reason is spinning so fast,” says Callingham.

Thankfully for us, gamma-ray bursts don’t explode out in all directions. They’re highly-focused events with all the power beamed into two narrow jets, and Apep doesn’t appear to be aimed at the Earth.

If the Earth did happen to be in the path of such a phenomenon, it could deplete the ozone layer and expose the planet to high doses of radiation. In the most unfortunate circumstances, this could trigger widespread extinction. There’s even speculation that gamma-ray bursts may have been responsible for minor extinction events in the past.

“Ultimately, we can’t be certain what the future has in store for Apep,” Tuthill says. “The system might slow down enough so it explodes as a normal supernova rather than a gamma-ray burst.

“However, in the meantime, it is providing astronomers a ringside seat into beautiful and dangerous physics that we have not seen before in our galaxy.”